The invention relates to a DC-DC converter which is configured as a phase-shifted tuft-bridge converter, i.e. as a full-bridge push-pull converter with phase-modulated actuation.
A DC-DC converter of this type has four semiconductor switches connected together to form a tuft-bridge for converting an input DC voltage into an AC voltage, a transformer to which, on the primary winding side, the AC voltage is applied, and a rectifier for rectifying the secondary voltage of the transformer. The AC voltage generated by the inverter alternatingly assumes an upper voltage level during a first active phase and during a second active phase, a lower voltage level. Between two successive active phases, the transformer is operated in a freewheeling phase in which the transformer is short-circuited on the primary side.
As well as the known advantages, in particular the zero-voltage switching, such a DC-DC converter also has disadvantages. For example, during the freewheeling phases, a freewheeling current, which is to enable the zero-voltage switching, flows in the primary side. The amount of the freewheeling current which is required to achieve the zero-voltage switching depends upon the input DC voltage. The amount of the freewheeling current that eventually becomes established depends, however, on the load current. According to the input DC voltage and the load current, a freewheeling current can therefore become established, the amount of which is significantly greater than would be required to achieve the zero-voltage switching. As a result, the primary side effective current is very large, which can cause significant conductive losses. Similarly, a free-wheeling current can become established which is insufficient to enable the zero-voltage switching, which can cause significant switching losses.
Furthermore, on the secondary side, the leakage inductances of the transformer and the parasitic capacitances of the rectifier result in an oscillation-susceptible system. By means of reverse currents in the rectifier, an oscillation thus arises, the amplitude of which, depending on the structure of the DC-DC converter, can lie far above the transformed primary voltage and which can be sustained over the entire active phase.
US 2015/0214847 A1 discloses a DC-DC converter in which between the DC terminals of a circuit, a smoothing capacitor and a voltage clamping circuit are connected. The voltage clamping circuit has a circuit device and a clamping capacitor.
DE 10 2015 012 343 A1 discloses a DC-DC conversion device of a motor vehicle, which is configured to convert an input DC voltage provided by an electrical energy source of the motor vehicle into a pre-determined output DC voltage for an on-board network of the motor vehicle. The DC-DC conversion device comprises, for increasing and/or smoothing the input DC voltage, an impedance source with two coils and two capacitors.
EP 2 258 218 A2 discloses a DC supply with a resonant circuit on a secondary side of a transformer for suppressing a surge voltage during the restoration of the output of diodes which form a rectifier circuit.
U.S. Pat. No. 6,388,896 B1 discloses a DC-DC converter with an input induction winding, a middle induction winding and an output induction winding which are arranged on a common magnetic core.
It is an object of the invention to provide an improved DC-DC converter which is configured as a phase-shifted full-bridge converter, and a method for operating it.